1. Field of the Invention
The present invention generally relates to an internal combustion engine having a hydraulic control system for controlling the operation of a variable camshaft timing (VCT) system of the type in which the camshaft position is circumferentially varied relative to the position of a crankshaft in reaction to oil pressure. In such a VCT system, an electro-hydraulic control system is provided to effect the repositioning of the camshaft and a locking system is provided to selectively permit or prevent the electro-hydraulic control system from effecting such repositioning.
More specifically, this invention relates to a multi-position VCT system actuated by engine oil pressure and having a locking piston mounted to a rotor, wherein the locking piston prevents oscillation of the rotor in an advance position, a retard position, and multitude of positions therebetween.
2. Description of the Prior Art
It is known that the performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft-powered chain drive or belt drive. It is also known that the performance of an internal combustion engine having dual camshafts, or but a single camshaft, can be improved by changing the positional relationship of a camshaft relative to the crankshaft.
It is also known that engine performance in an engine having one or more camshafts can be improved, specifically in terms of idle quality, fuel economy, reduced emissions, or increased torque, by way of a VCT system. For example, the camshaft can be "retarded" for delayed closing of intake valves at idle for stability purposes and at high engine speed for enhanced output. Likewise, the camshaft can be "advanced" for premature closing of intake valves during mid-range operation to achieve higher volumetric efficiency with correspondingly higher levels of torque. In a dual-camshaft engine, retarding or advancing the camshaft is accomplished by changing the positional relationship of one of the camshafts, usually the camshaft that operates the intake valves of the engine, relative to the other camshaft and the crankshaft. Accordingly, retarding or advancing the camshaft varies the timing of the engine in terms of the operation of the intake valves relative to the exhaust valves, or in terms of the operation of the valves relative to the position of the crankshaft.
Heretofore, many VCT systems incorporating hydraulics included an oscillatable rotor secured to a camshaft within an enclosed housing, where a chamber is defined between the rotor and housing. The rotor includes vanes mounted outwardly therefrom to divide the chamber into separated first and second fluid chambers. Such a VCT system often includes a fluid supplying configuration to transfer fluid within the housing from one side of a vane to the other, or vice versa, to thereby oscillate the rotor with respect to the housing in one direction or the other. Such oscillation is effective to advance or retard the position of the camshaft relative to the crankshaft. These VCT systems may either be "self-powered" having a hydraulic system actuated in response to torque pulses flowing through the camshaft, or may be powered directly from oil pressure from an oil pump. Additionally, mechanical connecting devices are included to lock the rotor and housing in either a fully advanced or fully retarded position relative to one another.
Unfortunately, the above VCT systems may have several drawbacks. For example, U.S. Pat. No. 4,858,572 to Shirai et al., teaches use of spring-loaded locking pistons in two circumferential positions to lock the rotor and housing in both a fully advanced and a fully retarded position. Shirai et al. discloses a first pin extending into a radial bore of the housing. The pin is urged radially inwardly toward the rotor by a spring mounted between the pin and the bore. When the VCT is in the fully retarded position, an upper end of the pin fits into a large radius portion of a radial hole in the rotor. If the VCT is changed to the advanced condition, the first pin is retracted from the radial hole by fluid pressure overcoming the spring. Another pin positioned opposite the first pin similarly locks the rotor in the fully advanced position. Thus, the rotor is prevented from rotary movement relative to the housing.
One drawback with Shirai et al. is that the pins act to lock the rotor relative to the housing in only two circumferential positions, either fully advanced or fully retarded. Another drawback is that the pins may stick in either the fully advanced or fully retarded position thus jamming the VCT. When the VCT changes from one position to another, part of the fluid pressure being transferred to the first and second fluid chambers gets redirected to one of the pins to retract the pin. Accordingly, the fluid pressure is applied simultaneously to the fluid chambers and the pin. When the fluid pressure in the fluid chambers is sufficient to start rotating the rotor before the pin is fully retracted, the rotor side loads the pin causing the pin to stick in the radial hole and thus renders the VCT inoperative.
U.S. Pat. No. 5,836,275 to Sato recognized the above-mentioned problem with Shirai et al. and attempted a solution. Sato teaches use of hydraulic strategy to retract the pin before charging either the first or second fluid chambers. Accordingly, Sato discloses fluid pressure supplied to the radial hole in the rotor while simultaneously charging fluid passages communicating with either the first or second fluid chambers. Because the fluid passages are initially restricted, and thus only partially in communication with the first or second fluid chambers, the fluid pressure acts primarily on the pin to retract the pin before any appreciable rotation of the rotor occurs. After the pin retracts, the rotor rotates enough to permit the passages to overcome their restriction and fully communicate with the fluid chambers to effect rotation of the rotor. Regrettably, however, the Sato invention permits locking of the rotor in only the fully retarded position.
U.S. Pat. No. 5,797,361 to Mikame et al. recognized another problem with Shirai et al. That is, in the retracted position, the upper end of the pin loads an external surface of the rotor due to the spring force pushing the pin toward the rotor. This wears the rotor's circumference creating grooves that facilitate increased leakage between the housing and the rotor beyond an acceptable level. The leakage lowers the oil pressure in the chamber and thus deteriorates the responsiveness of the VCT. In addition, the wear condition hinders smooth relative rotation between the rotor and housing. Finally, Mikame et al. submits that maintaining fluid pressure in the Shirai et al. invention at a certain level is difficult, since Shirai et al. relies on fluid pressure caused by torque fluctuations in the camshaft. Unfortunately, Mikame et al. suffers from the same drawback as Sato. That is, the locking piston locks the rotor relative to the housing in only one circumferential position. Finally, the locking piston of Mikame et al. and the hole with which it interlocks have clearance therebetween that permits circumferential free play or slack between the housing and the rotor. This slack condition could lead to noise at engine startup as the locking piston is knocked about within the hole.
Accordingly, all of the above mentioned prior art references incorporate a locking piston mounted within a housing and lockable with a rotor in only one position per locking piston. For example, the Shirai et al. reference is lockable in only a fully advanced or fully retarded position using two locking pistons. Furthermore, each locking piston interlocks with the rotor in diametral engagement, which may lead to sticking conditions of the VCT, as discussed in Sato.
Therefore, what is needed is a VCT system that is designed to overcome the problems associated with prior art variable camshaft timing arrangements using locking pistons, by providing a variable camshaft timing system that locks a rotor and housing together in more than one position per locking piston, is not susceptible to unintended lock-up conditions created by diametral jam conditions between the locking piston and a locking piston hole, and does not permit rotational slack between the rotor and housing.